1. Field of the Invention
The present invention relates to a battery, a camera and a camera system, and in particular, it relates to a technology that provides a user with accurate information indicating whether or not the battery needs to be refreshed.
2. Description of the Related Art
Cameras and the like are normally equipped with a battery check function through which the voltage at the source battery is monitored over specific time intervals and different warning messages can be issued when the battery voltage becomes lower than one of several predetermined reference levels. The warning, which may indicate “sufficient battery power remaining”, “remaining battery power low”, “need to replace battery” or the like, may be displayed on, for instance, a liquid crystal display unit by using icons. Such a battery check function is provided simply for the purpose of prompting the photographer to replace the battery. Thus, the remaining battery power is not indicated with accuracy and the user is not provided with information indicating the precise number of pictures that can be taken, the precise length of time over which the camera can be operated or the like on the remaining battery power.
Japanese Laid Open Patent Publication No. H9-297166 discloses a method for indicating the remaining battery power. In this method, a current detector, a voltage detector and a communication circuit are included in a secondary battery pack, remaining battery power information, charge/discharge current detection information and battery cell voltage detection information originating in the battery pack are received at an electronic device (such as a video camera) and the level of the currently available battery power is calculated and indicated based upon the received information. Compared to the battery check described earlier, the remaining battery power can be ascertained with a higher degree of accuracy and the length of time over which the camera can be operated or the like can be accurately indicated to the user by adopting this method.
However, the absolute capacity of a secondary battery is bound to become lower after repeated use under normal circumstances. For this reason, the older the battery becomes, the greater the deviation of its actual absolute capacity relative to the absolute capacity recognized by the electronic device, and the remaining battery power can no longer be determined accurately through the method described above. In order to eliminate such a deviation, the battery needs to undergo a so-called refresh (calibration) operation through which it is completely discharged. The charge quantity of a battery that has been first refreshed and then fully charged equals the absolute capacity of the battery. While chargers with a refresh function have become available for practical applications in recent years, it takes a long time to perform a single refresh operation (normally longer than a charge operation), and having to perform a refresh operation frequently is bound to inconvenience the user. This problem may be adequately addressed with a function that provides the user with accurate information indicating whether or not the battery needs to be refreshed.